My PC shutdown automatically!! Install video card to solve the issue?

My PC shutdown automatically but the green LED light is on. When I press restart button, nothing happens. I need to press the power button for 5 seconds to totally shut it down. Then press the power button again to on the computer.

During Windows loading or sometimes on the Windows Log-in my computer shutdown again but the green LED light is on and I need to do the same thing to shut it off and turn it on.

I don't know the exact problem is. The last things that I did were:
1. I upgrade my RAM from 256MB to 1GB
2. Upgrade my Windows to Service Pack 2.

I brought my computer to the Computer Service Center for check up. And they said that to resolve the issue:
1. They need to install external video card.
2. There's a problem with my motherboard.